1. Field of the Invention
The present invention concerns an alkaline electrolyte secondary electric cell, in particular a nickel-hydridable metal cell (Nixe2x80x94MH).
2. Description of the Prior Art
The main advantage of secondary cells is their ability to store energy. However, a completely charged cell which is not used rapidly loses part of its charge. Nickel-cadmium (Nixe2x80x94Cd) storage cells, for example, have long been used as an autonomous energy source. They are known to have good charge retention, i.e. when stored in the charged state the capacity falls slowly. The charge lost by a completely charged Nixe2x80x94Cd cell is about 20% over 7 days at 40xc2x0 C.
Since modern portable appliances require ever more powerful autonomous energy sources, a new cell has recently been developed. This is the nickel-hydridable metal storage cell (Nixe2x80x94MH). Such a storage cell has a specific energy which is at least equal to that of the Nixe2x80x94Cd storage cell but the self-discharge rate is high and thus the user is greatly inconvenienced. The charge lost by a Nixe2x80x94MH storage cell which is stored in its completely charged state is about twice as much as that for a Nixe2x80x94Cd storage cell, i.e. 40% over 7 days at 40xc2x0 C. This poor result is due to the fact that the MH electrode, once charged, has a higher reducing character than the Cd electrode.
Self-discharge is generally attributed in part to nitrogen-containing shuttles. Ammonia and nitrites present in the storage cell are oxidized to nitrates at the positively charged electrode, discharging it. Also, nitrates and nitrites are reduced to ammonia at the negatively charged electrode, discharging that as well. Those reactions can occur a number of times since the species generated at the positive electrode will react at the negative electrode where they are transformed into species which are capable of reacting at the positive electrode. This is why they are called shuttles.
Reduction of nitrates and nitrites to ammonia is accelerated at an MH electrode. If such a reaction is the rate limiting step in the kinetics of the nitrogen-containing shuttle, then over a given period more shuttles can be produced in an Nixe2x80x94MH storage cell. This hypothesis is generally accepted as the explanation for the high self-discharge rate in Nixe2x80x94MH cells (Ikoma et al., J. Electrochem. Soc., 143, 6, 1996, 1904-1907).
In a cell, the positive and negative electrodes are separated by an insulative material which assures ionic conduction while preventing electrical contact between the two electrodes. In order to maintain electrical insulation between the two electrodes, the separator must be mechanically and chemically stable under service conditions. It must retain its properties during the entire service life of the cell. Further, a high ionic conductivity requires that the separator be uniformly wetted by the electrolyte.
In order to limit the influence of the nitrogen-containing species, is has been proposed to replace the polyamide separator generally used in Nixe2x80x94MH storage cells by a polyolefin separator with a higher chemical stability in that medium. A polyamide separator is a potential source of nitrogen-containing impurities due to its deterioration in the highly alkaline electrolyte used in Nixe2x80x94MH storage cells (U.S. Pat. No. 5,278,001).
The most frequently used separators at this time are based on polyethylene and/or polypropylene. Separators composed of fibers with a polypropylene core surrounded by a polyethylene sleeve are known in themselves, for example. However, polyethylene separators are difficult to wet in an aqueous electrolyte. In order to improve wettability, manufacturers have turned towards using separators which are grafted with hydrophilic monomers, which are generally vinyl compounds. The grafting method which has proved to be the most effective employs ionizing radiation, with the irradiation and grafting being carried out in two steps or simultaneously.
Such separators introduce no or very few nitrogen-containing species, but the other electrochemical components present in the cell remain unwanted sources of nitrogen-containing compounds. Thus self-discharge of a Nixe2x80x94MH storage cell remains very much higher than that observed under the same conditions for a Nixe2x80x94Cd storage cell.
The aim of the present invention is to propose a separator which increases charge retention in alkaline electrolyte secondary cells, in particular nickel-hydridable metal type cells.
The present invention consists in a secondary electric cell comprising at least one positive electrode and one negative electrode positioned either side of a separator composed of fibers of a polyolefin grafted with a vinyl monomer and containing means for absorbing and retaining nitrogen in a strongly basic medium, with a pH of at least 12, said means being constituted by said separator.
It has been established that, surprisingly, apart from the fact that it does not generate nitrogen-containing species, the separator of the present invention efficiently traps nitrogen-containing species originating from the other electrochemical components. It can therefore substantially reduce the quantity of nitrogen-containing shuttles which contribute to self-discharge of Nixe2x80x94MH storage cells and therefore increase their charge retention.
The separator has an ability to absorb and retain nitrogen in a strongly basic medium, with a pH of at least 12, in a proportion of at least 3xc3x9710xe2x88x924 moles of nitrogen per gram of separator when the fibers are constituted by at least two polyolefins.
The separator may, for example, be constituted by fibers with a polypropylene core which is surrounded by a sleeve of polyethylene or a mixture of these fibers with fibers which contain only polyethylene.
The separator can absorb and retain nitrogen in a strongly basic medium, with a pH of at least 12, in a proportion of more than 5xc3x9710xe2x88x924 moles of nitrogen per gram of separator when said fibers are constituted by a single polyolefin.
The polyolefin is preferably selected from polyethylene and polypropylene. These polymers have the advantage of having high chemical stability in an alkaline medium.
The vinyl monomer is preferably selected from acrylic acid and methacrylic acid. Grafting with hydrophilic groups enhances the wettability of the separator.
The present invention also consists in a process for the production of a separator which can absorb and retain nitrogen in a strongly basic medium, composed of polyolefin fibers grafted with a vinyl monomer, the process comprising the following steps:
impregnating a porous separator composed of ungrafted polyolefin fibers with an aqueous solution containing a vinyl monomer by forcing the solution to penetrate into all of the pores of the separator, such that the volume of the solution retained by the separator after impregnation is at least equal to the pore volume of the separator (xe2x89xa7100% of the pore volume);
positioning the impregnated separator between two films of polyolefin with no gas being present between the surface of the impregnated separator and the surface of the film;
irradiating the assembly constituted by the separator and the films with ultraviolet radiation to graft the monomer over the entire surface of each of the fibers;
rinsing and drying the grafted separator.
Merely immersing the separator in the solution is insufficient for the solution containing the monomer to bathe the entire surface of each fiber and to be distributed homogeneously over the entire length of the fibers to the core of the separator. The solution must be forced to occupy all of the pores of the separator. This can be achieved by drawing the solution through the separator, for example using a suction pump.
The solution preferably also contains a grafting initiator, for example benzophenone.
Further, the separator must be constantly bathed in the monomer solution for the entire duration of the grafting operation. Loss of solution which may occur during this step, for example by evaporation, must therefore be limited.
The volume of the solution retained by the separator after impregnation and irradiation is preferably at least equal to the pore volume of the separator (xe2x89xa7100% of the pore volume).
In an advantageous variation, the solution is circulated between the separator and the films during irradiation. Thus the separator is surrounded by solution and loss through evaporation is prevented. A reserve of fresh solution containing the monomer to be grafted is always in contact with the separator and depleted solution is evacuated.
Rinsing is preferably carried out using deionized water. Solvents which are difficult to eliminate and/or liable to react chemically with the grafted separator must be avoided.
In a separator produced by the process described above the monomer is homogeneously grafted over the entire surface of each of the fibers.
The invention further consists in a nickel-hydridable metal secondary electric cell comprising at least one positive electrode and one negative electrode positioned either side of a separator produced by the process described above.
In a first embodiment of the invention, the polyolefin fibers are constituted by a mixture of polyethylene and polypropylene and the monomer is acrylic acid, the separator being capable of absorbing and retaining nitrogen in a proportion which is at least 3xc3x9710xe2x88x924 moles of nitrogen per gram of separator.
In a second embodiment of the invention, the polyolefin fibers are constituted by a single polyolefin selected from polyethylene and polypropylene and the monomer is acrylic acid, the separator being capable of absorbing and retaining nitrogen in a proportion which is at least 5xc3x9710xe2x88x924 moles of nitrogen per gram of separator.
Further features and advantages of the present invention will become apparent from the following examples which, of course, are given by way of non-limiting illustration and with reference to the accompanying drawings.